The present invention relates to a liquid crystal display device, and relates to a light source suitable for efficiently enhancing a brightness of a display screen and making it uniform over the entire display screen area, and a control method therefor.
A display device using a liquid crystal display element (also called a liquid crystal display panel), an electroluminescent element (which is divided into an organic system and an inorganic system depending on a fluorescent material used, hereinafter referred to as an EL element), a field emission device (hereinafter referred to as an FE element) or the like displays an image without requiring a space (a vacuum envelope) for scanning an electron beam two-dimensionally on the back of the display screen as in a cathode ray tube (CRT). Accordingly, these display devices have characteristics that they are thin and light as compared with the CRT, and power consumption is low. These display devices are sometimes called a flat panel display because of its external appearance.
The display device using a liquid crystal display element, an EL element or a field emission device or the like has been widely spread due to the above-described advantage with respect to the CRT in place of a display device using the CRT in various uses. The fact that replacement from the CRTs to the flat panel displays has been progressed is also due to a technical innovation in enhancement of quality of images of a liquid crystal display element or an EL element. With the recent spreading of multi-media or the Internet, displaying of moving pictures has been strongly demanded. For example, in the display device using a liquid crystal display element, an improvement in a liquid crystal material or a driving method has been made for realizing a moving picture display. However, in the display device called a flat panel display as well as a display device using a liquid crystal display element, an increase of brightness is an important factor for displaying an image equal in quality to that of a conventional CRT.
For obtaining a moving picture display equal in quality to that of the CRT, it is essential to have impulse-type light generation as by scanning an electron beam projected from an electron gun on each pixel to excite phosphors of respective pixels to luminescence. On the other hand, for example, the liquid crystal display device utilizes the hold-type light generation using a display signals, and is repeated every image display period of one frame (a period for transferring video signals to all pixels of a display screen). (See “LIQUID CRYSTAL”, Vol. 3, No. 2 (1999), p. 99-p. 106.)
On the other hand, there is a technique for modulating brightness of a light source according to a scene of a moving picture signals transmitted to the liquid crystal display device. In this technique, a maximum brightness data, a minimum brightness data and an average brightness data of a video signal transmitted to the liquid crystal display device are read every image (in the case of a movie film, every “frame”) constituting a moving picture frame to control a current (hereinafter called a lamp current) supplied to a light source according to the data. Suppose a current supplied normally to the light source is a reference current (for example, 4.5 mA), in the case of an image which is bright over the entire area, a lamp current is set to be higher than the reference current (for example, 8 mA) in a certain period, and is returned to the reference current later. Conversely, in the case of an image which is dark over the entire area, a lamp current is set to be lower than the reference current (for example, 1.5 mA). (See “NIKKEI ELECTRONICS”, Nov. 15, 1999 issue, No. 757, 1999, p 139-p 146)
In the case of the former (the wholly bright image), temperature rise of a light source is larger by a portion corresponding to an increase in current supplied to the light source from the reference current. In the case of a fluorescent lamp, vapor pressure of mercury (Hg) within a fluorescent lamp rises due to rising of temperature thereof, and mercury atoms (the amount of mercury vapor) increase within the fluorescent lamp. On the other hand, surplus mercury atoms are present within the fluorescent lamp, there is increased probability that ultraviolet rays produced within the fluorescent lamp due to collision between hydrogen atoms and electrons are absorbed by the mercury atoms, and brightness of the fluorescent lamp decreases. For avoiding this influence, a lamp current is set to be higher than the reference current in the period described above, after which the lamp current is returned to the reference current before the mercury vapor pressure within the fluorescent lamp changes. By changing the lamp current as described above, the brightness of the fluorescent lamp is made higher than that when the reference current is supplied thereto.
In the case of the latter (the wholly dark image), when the brightness of the light source is high, it is necessary to suppress a leakage of a small amount of light from a pixel which displays black or a color close thereto. In the wholly dark screen, even for the pixel whose light transmission is set to be highest within the screen, the absolute amount of light to be transmitted is small. Because of this, the lamp current is set to be lower than the reference current, and the brightness of the light source is suppressed to restrict leakage of light from a pixel which displays black or color close thereto, and power consumption in the light source is reduced.
From a combination of the two techniques, the dynamic range of brightness (the ratio of the maximum brightness to the minimum brightness) in the moving picture image as a whole becomes 2.8 times that of the conventional one, and the contrast ratio is from 400:1 to 500:1, which is not less than 2 times that of the conventional liquid crystal display device.